A conventionally known oil ring scrapes off excess engine oil attached to a cylinder inner wall surface of an internal combustion engine, and forms an appropriate oil film, to thereby prevent seizure of a piston resulting from operation of the internal combustion engine, and reduce wear of a slide contact surface between the oil ring and the cylinder. Various forms of such an oil ring have been known. For example, the following Patent Literature describes a combined oil ring including a pair of upper and lower side rails and a spacer expander arranged therebetween, in which working surfaces of the side rails, in cross-section, follow the asymmetrical shape of a polynomial of the second order in a first segment, with h(x)=ax+bx2, and after passing a supporting vertex h(x) configured as an edge, in a third segment follow the asymmetrical shape of the function h(x)=cx2, with c as a multiple of b.